1. Field of the Invention
The present invention relates to a reflective mask blank for EUV (Extreme Ultra Violet) lithography (hereinbelow, referred to as “EUV mask blank” in this Description), a reflective mask for EUV (hereinbelow, referred to as “EUV mask” in this Description) and a substrate with a reflective layer, applied thereto, which are used in semiconductor manufacturing or the like.
2. Discussion of Background
In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for writing, on a Si substrate or the like, a fine pattern, which is required for writing an integrated circuit comprising such a fine pattern. However, the conventional exposure techniques using light exposure have been close to the limit while semiconductor devices have had finer patterns at an accelerated pace. In the case of light exposure, it is said that the resolution limit is about ½ of exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a short wavelength than ArF lasers, has considered as being promising as an exposure technique for 45 nm or below. In this Description, it should be noted that the EUV light means a ray having a wavelength in a soft X ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
It is impossible to use EUV light in conventional dioptric systems as in photolithography using visible light or ultraviolet light since EUV light is apt to be absorbed by any substances and since substances which absorb EUV light have a refractive index close to 1. For this reason, a catoptric system, i.e., a combination of a reflective photomask and a mirror, is employed in EUV light lithography.
A mask blank is a laminated member for fabrication of a photomask, which has not been patterned yet. When a mask blank is used for a reflective photomask, the mask blank has a structure wherein a substrate made of glass or the like has a reflective layer for reflecting EUV light and an absorbing layer for absorbing EUV light formed thereon in this order. The reflective layer is normally a multilayer film, which comprises high refractive layers and low refractive layers alternately laminated to increase a light reflectance when irradiating a film surface with a ray, more specifically when irradiating a film surface with EUV light. The absorbing layer comprises a material layer, which contains a material having a high absorption coefficient in connection with EUV light, as the main component (for example, see Patent Document 1).
The reflective layer and the absorbing layer are deposited by ion beam sputtering or magnetron sputtering. When the reflective layer and the absorbing layer are deposited, the substrate is supported by a chucking and supporting means in a film deposition chamber. Although there are a mechanical chuck and an electrostatic chuck as the chucking and supporting means, an electrostatic chuck is preferably used because of a reduction in the generation of dust during chucking or another reason. However, when the substrate has a low conductivity as in a glass substrate, there is a risk that dielectric breakdown is caused since a high voltage is required to be applied in order to obtain a chucking force at the same level as, e.g., a silicon wafer.
In order to solve such a problem, Patent Documents 2 and 3 have proposed forming a chucking layer for chucking and supporting a substrate by an electrostatic chuck (hereinbelow, also referred to as the chucking layer). These documents have described that the above-mentioned chucking layer comprises a conductive film made of a material having a higher conductivity than glass substrates, such as Si, Mo, Cr, chromium oxynitride (CrON) or TaSi.
Further, Patent Document 4 has described that the chucking layer may comprise a film made of, e.g. BaTiO3, or PZT (PbZr1-xTixO3), which causes dielectric polarization by application of an electric field, in addition to the above-mentioned conductive films.
Patent Document 1: JP-A-2002-319542 (U.S. Pat. No. 6,749,973)
Patent Document 2: JP-A-2003-501823
Patent Document 3: JP-A-2005-210093
Patent document 4: JP-A-2000-208594